1. Field of the Invention
The present invention relates to an eye examining instrument, and more particularly to a system for focusing the instrument to the ground of the eye to be examined.
2. Description of the Prior Arts
In a conventional eye-ground camera, the focusing to the eye ground has been achieved by adapting the sight of the eye to cross-hairs in the finder and by adjusting the photographing lens in such a manner that the eye ground can be clearly viewed in this state. However such focusing method inevitably involves personal fluctuation and is difficult to conduct rapidly.
U.S. Pat. No. 3,016,000 discloses a method of focusing the camera to the eye ground by projecting a focus mark onto the eye ground and adjusting the focusing lens so as that the focus mark can be clearly viewed. Also U.S. Pat. No. 3,925,793 (German Pat. No. 2,415,319) and U.S. patent application Ser. No. 945,845 disclose a focusing method wherein plural marks are projected on the eye ground and the marks are aligning under observation. Although rapid focusing has been rendered possible by these methods, there still remains a strong demand for automatic focusing of the camera, as the setting of an eye-ground camera simultaneously requires the alignment of the eye axis with the optical axis of the objective lens, the distance adjustment between the cornea and the objective lens, and the focus adjustment. The operator has to constantly pay attention to these three factors as the above-mentioned alignment and distance are easily affected by small movements of the subject to be examined while the focusing is affected by a change in the sight of the eye to be examined. For this reason automatic focusing, if realized, will significantly alleviate the load to the operator and contribute to the probability of obtaining photographs of improved image quality. Also in case of continuous photographing with successively displaced viewing fields, it becomes necessary to correct the focusing for each field displacement, and an automatic focusing will contribute to the image quality in such case.
A pioneer invention for automatic focusing of the eye-ground camera to the eye ground is disclosed in U.S. Pat. No. 3,614,214, in which a dichroic mirror reflecting the infrared light but transmitting the visible light is provided in front of an ordinary eye-ground camera in an oblique position to deflect the detecting beam from an automatic optometer toward the eye to be examined and to again deflect the reflected beam from the eye to said optometer, whereby the focusing lens of the eye-ground camera is adjusted by the output of said optometer. In case, however, of adopting a wider photographing angle there is required a shorter working distance between the camera and the eye, and the presence of the dichroic mirror in front of the camera will hinder the use of a wider photographing angle. Also the increased diameter of the photographing beam required in case of a wider photographing angle requires a longer dichroic mirror, thus leading to a drawback of an increased working distance between the eye and the camera.
The above-mentioned method is advantageous in that the focusing can be achieved without the artificial mydriasis induced by the use of a mydriatic as said optometer utilizes, as the detecting beam, an infrared light not reactive to the pupil. However, the determination of the photographing field, which is required in addition to the focusing in the eye-ground photographing, cannot be achieved by projecting an illuminating light in the infrared wavelength region from the eye-ground camera since it is deflected by said dichroic mirror and does not reach the eye to be examined.
In addition, the use of this eye ground camera is characterized by difficulties specific thereto since the illumination of the eye ground and the observation of the reflected beam occur through the small aperture of the pupil, and a part of the illuminating beam may be reflected by the cornea of the eye to be examined.